1. Field of the Invention
The present invention relates generally to the field of optical communications. More specifically, the present invention discloses an optical wavelength add/drop multiplexer for use in wavelength division multiplex (WDM) optical communications.
2. Statement of the Problem
Optical wavelength division multiplexing has gradually become the standard backbone network for fiber optic communication systems. WDM systems employ signals consisting of a number of different wavelength optical signals, known as carrier signals or channels, to transmit information on optical fibers. Each carrier signal is modulated by one or more information signals. As a result, a significant number of information signals may be transmitted over a single optical fiber using WDM technology.
Despite the substantially higher fiber bandwidth utilization provided by WDM technology, a number of serious problems must be overcome, for example, multiplexing, demultiplexing, and routing optical signals, if these systems are to become commercially viable. The addition of the wavelength domain increases the complexity for network management because processing now involves both filtering and routing. Multiplexing involves the process of combining multiple channels (each defined by its own frequency spectrum) into a single WDM signal. Demultiplexing is the opposite process in which a single WDM signal is decomposed into individual channels. The individual channels are spatially separated and coupled to specific output ports. Routing differs from demultiplexing in that a router spatially separates the input optical channels to output ports and permutes these channels according to control signals to a desired coupling between an input channel and an output port.
The Applicants"" U.S. Pat. No. 5,724,165 and U.S. patent application Ser. No. 08/739,424 (Kuang-Yi Wu et al.) teach two independent methods for high performance signal routing (U.S. Pat. No. 5,724,165) and wavelength de-multiplexing (Ser. No. 08/739,424). In U.S. Pat. No. 5,724,165, new structures for realizing optical switches (routers) were disclosed that achieve very high extinction ratios. However, these switches are wavelength independent. In Ser. No. 08/739,424, a system is disclosed to provide the functions of wavelength de-multiplexing and routing. However, this single stage design relies primarily on the filter design. The transmission function of the filter has to be close to an ideal square flat-top to realize the desired low crosstalk operation.
Other prior art in the field includes the following:
Ammann, xe2x80x9cSynthesis of Electro-Optic Shutters Having A Prescribed Transmission vs. Voltage Characteristic,xe2x80x9d Journal of the Optical Society of America, vol. 56, no. 8, pp. 1081-1088 (August 1966)
Harris et al., xe2x80x9cOptical Network Synthesis Using Birefringent Crystals* I. Synthesis of Lossless Networks of Equal-Length Crystals,xe2x80x9d Journal of the Optical Society of America, vol. 54, no. 10, pp. 1267-1279 (October 1964)
Patel et al. disclose an optical switch using a series of birefringent layers and ferroelectric cells to route an input beam to any of a plurality of output positions.
Glance discloses a tunable add/drop filter using a 1xc3x97N optical switch, a wavelength grating router (WGR), and a multiplexer. The WGR outputs include a set of retain outputs that are coupled directly to the multiplexer and a drop output. The particular WDM frequency component that is routed to the drop output is determined by the WGR input port at which the WDM signal is received. The 1xc3x97N switch provides the WDM signal to the proper WGR input so that a selected frequency is provided to the drop output. The retained signals and any added signals are multiplexed by the multiplexer.
DeJule et al. disclose an optical switching device using a plurality of polarization-independent switching cells arranged in matrix form. Each switching cell consists of a spatial light modulator and a number of polarized beamsplitters that can be used to selectively direct an input optical beam along either of two axes.
Nelson discloses an optical switch employing an electro-optical crystal that exhibits birefringence in each of two different light paths when the crystal is disposed in orthogonally-oriented electric fields. Each light path is sensitive to a different one of the two electric fields and has its own set of fast and slow axes.
Meadows discloses a 2xc3x972 electro-optical switch that employs dielectric film polarizing beamsplitters and a switchable electro-optic retarder.
Ammann and Harris et al. provide general background in the field of optical filter design.
3. Solution to the Problem
None of the prior art references discussed above show an optical wavelength add/drop multiplexer that uses a network of wavelength slicers to separate the input WDM channels from a first optical link into a plurality of sets of channels. At least one set of channels is then separated into individual channels by an interference filter to interface with an add/drop switch array. The output channels from the add/drop switch array and a selected set of channels from the wavelength slicer network can be combined and transmitted over a second optical link.
This invention provides an optical wavelength add/drop multiplexer for communications between two optical links supporting wavelength division multiplexing (WDM). A wavelength slicer spatially separates the input signal into two sets of channels. An optical filter, such as an interference filter, spatially separates the a subset of the input channels into an array of separated channels. A programmable optical add/drop switch array selectively routes channels from an array of input ports to an array of drop ports, substitutes channels from an array of add ports in place of the dropped channels, and routes the remaining input channels and added channels to an array of output ports. The channels from the output ports of the said add/drop switch array are then combined and transmitted into the second optical link. A network of wavelength slicers can be used to spatially separate the input signal into a larger number of sets of channels that can either be accessed by a number of add/drop switch arrays, or pass unchanged as xe2x80x9cexpress lanesxe2x80x9d to the second optical link. In an alternative embodiment, a circulated drop filter consisting of an optical circulator and a series of fiber Bragg gratings is used to select a predetermined series of input channels to be processed by the add/drop switch array, with the remaining channels being passed by the circulated drop filter as express lanes.
A primary object of the present invention is to provide an optical wavelength add/drop multiplexer that can separate multiple channels from an input WDM signal and selectively substitute channels from series of add ports in place of the input channels.
Another object of the present invention is to provide an optical wavelength add/drop multiplexer that can be use to augment the channel capacity of an existing central office equipment for optical communications.